Dynamic impedance control for input/output buffers

Abstract

A system and method of performing off chip drive (OCD) and on-die termination (ODT) are provided. A common pull-up network composed of transistors and a common pull-down network composed of transistors are employed to implement both of these functions. In drive mode, the pull-up network is configured to produce a calibrated drive impedance when an “on” output is to be generated, and the pull-up network is configured to produce a calibrated drive impedance when an “off” output is to be generated. In termination mode, the pull-up network and the pull-down network are configured to produce a calibrated pull-up resistance and pull-down resistance respectively such that together, they form a split termination.

1. An apparatus comprising:
 a first combined drive and termination circuit configured to operate with a first supply voltage, the first combined drive and termination circuit comprising:
 a variable impedance pull-up network; 
 a variable impedance pull-down network; 
 at least one control input for setting a configuration of the pull-up network; and 
 at least one control input for setting a configuration of the pull-down network; 
 
 a controller that controls calibration being performed in four steps:
 1) pull-up network calibration for drive mode when a data output is logic high; 
 2) pull-down network calibration for drive mode when a data output is logic low; 
 3) pull-up network calibration for termination mode; and 
 4) pull-down network calibration for termination mode; and 
 
 a replica of at least part of the combined drive and termination circuit for use in performing calibration; 
 the apparatus having a termination mode of operation in which the variable impedance pull-up network is configured to have a pull-up network termination impedance and the variable impedance pull-down network is configured to have a pull-down network termination impedance, the pull-up network and the pull-down network in combination functioning as a split termination; 
 the apparatus having a drive mode of operation in which:
 to drive a high output, the pull-up network is configured to generate a specific impedance when switched ON; 
 to drive a low output, the pull-down network is configured to generate a specific impedance when switched ON; 
 each of the pull-up network and the pull-down network having a plurality of transistors sized to provide identical impedances, and 
 each of the pull-up network and the pull-down network being configured to have a desired impedance by progressively varying the number of switched ON transistors. 
 
 
     
     
       2. The apparatus of  claim 1  wherein the impedance of the pull-up network and the impedance of the pull-down network are configured using one or more thermometer codes. 
     
     
       3. The apparatus of  claim 2  wherein the number of switched ON transistors is progressively varied using said one or more thermometer codes. 
     
     
       4. The apparatus of  claim 3  further configured to convert a Gray code that encodes the number of switched ON transistors to a thermometer code of said one or more thermometer codes. 
     
     
       5. An arrangement comprising:
 core logic; 
 a plurality of I/Os (input/outputs), each having a respective I/O pad; and 
 for each I/O, a respective apparatus according to  claim 1 ; 
 the combined drive and termination circuits functioning to generate outputs from the core logic and to terminate external inputs for the core logic. 
 
     
     
       6. The arrangement of  claim 5  wherein the pull-up and pull-down networks are switched dynamically between two impedance settings when commutating between drive and termination modes. 
     
     
       7. The arrangement of  claim 5  wherein each apparatus further comprises:
 first pre-drivers connected to the variable impedance pull-up network for configuring the pull-up network by sending a first control signal to the at least one first control input, the first pre-drivers being configured to operate with a second supply voltage higher than the first supply voltage; and 
 second pre-drivers connected to the variable impedance pull-down network for configuring the pull-down network by sending a second control signal to the at least one second control input, the second pre-drivers being configured to operate with the second supply voltage. 
 
     
     
       8. The arrangement of  claim 7 , wherein:
 for each I/O, each first pre-driver of the respective combined drive and termination circuit comprises AND-OR-AND logic comprising a first AND gate that receives a first input to indicate drive high, and a second AND gate that receives a third input to indicate termination; 
 for each I/O, each second pre-driver of the respective combined drive and termination circuit comprises AND-OR-AND logic comprising a third AND gate that receives a second input to indicate drive low, and a fourth AND gate that receives the third input to indicate termination; and 
 for each I/O, the first pre-drivers and the second pre-drivers having outputs that cause the variable impedance pull-up network and the variable impedance pull-down network to switch between two impedance settings. 
 
     
     
       9. The apparatus of  claim 1  further comprising a calibration logic that calibrates the impedances against an impedance reference. 
     
     
       10. An arrangement comprising:
 core logic; 
 a plurality of inputs each having a respective input pad, and a plurality of outputs each having a respective output pad; 
 for each input pad, a respective apparatus according to  claim 1  in which the combined drive and termination circuit is permanently configured to be in termination mode; and 
 for each output pad, a respective apparatus according to  claim 1  in which the combined drive and termination circuit is permanently configured to be in drive mode. 
 
     
     
       11. An apparatus according to  claim 1  further comprising:
 a controller that generates the control inputs as a function of whether the first combined drive and termination circuit is in a drive mode or a termination mode. 
 
     
     
       12. The apparatus of  claim 1  wherein:
 the plurality of transistors in the pull-up network are connected together in parallel; and 
 the plurality of transistors in the pull-down network are connected together in parallel. 
 
     
     
       13. The apparatus of  claim 1  wherein the pull-up network comprises a plurality of P-type mosfet transistors, and the pull-down network comprises a plurality of N-type mosfet transistors. 
     
     
       14. The apparatus of  claim 1  further comprising:
 interconnections that pass common calibration values to each combined drive and termination circuit. 
 
     
     
       15. The apparatus of  claim 1  wherein the pull-up network comprises P-type transistors, and the pull-down network comprises N-type transistors, and wherein the interconnections deliver:
 a first calibration value that sets how many of the N-type transistors to enable for drive mode when a data output is logic low; 
 a second calibration value that sets how many of the P-type transistors to enable for drive mode when a data output is logic high; 
 a third calibration value that sets how many of the N-type transistors to enable for termination mode; and 
 a fourth calibration value that sets how many of the P-type transistors to enable for termination mode. 
 
     
     
       16. An apparatus comprising:
 a first combined drive and termination circuit configured to operate with a first supply voltage, the first combined drive and termination circuit comprising:
 a variable impedance pull-up network; 
 a variable impedance pull-down network; 
 at least one control input for setting a configuration of the pull-up network; and 
 at least one control input for setting a configuration of the pull-down network; and 
 
 a replica of at least part of the combined drive and termination circuit for use in performing calibration; 
 the pull-up network and the pull-down network being each formed entirely of P-type transistors or N-type transistors, the apparatus further comprising:
 a controller that controls calibration being performed in two steps:
 1) pull-up network calibration for drive mode when a data output is logic high; and 
 2) pull-up network calibration for termination mode; 
 
 
 the apparatus having a termination mode of operation in which the variable impedance pull-up network is configured to have a termination impedance and the variable impedance pull-down network is configured to have a pull-down network termination impedance, the pull-up network and the pull-down network in combination functioning as a split termination; 
 the apparatus having a drive mode of operation in which:
 to drive a high output, the pull-up network is configured to generate a specific impedance when switched ON; and 
 to drive a low output, the pull-down network is configured to generate a specific impedance when switched ON; 
 
 each of the pull-up network and the pull-down network having a plurality of transistors sized to provide identical impedances, and 
 each of the pull-up network and the pull-down network being configured to have a desired impedance by progressively varying the number of switched ON transistors. 
 
     
     
       17. The apparatus of  claim 16  wherein the impedance of the pull-up network and the impedance of the pull-down network are configured using one or more thermometer codes. 
     
     
       18. The apparatus of  claim 17  wherein the number of switched ON transistors is progressively varied using said one or more thermometer codes. 
     
     
       19. The apparatus of  claim 18  further configured to convert a Gray code that encodes the number of switched ON transistors to a thermometer code of said one or more thermometer codes. 
     
     
       20. An arrangement comprising:
 core logic; 
 a plurality of I/Os (input/outputs), each having a respective I/O pad; and 
 for each I/O, a respective apparatus according to  claim 16 ; 
 the combined drive and termination circuits functioning to generate outputs from the core logic and to terminate external inputs for the core logic. 
 
     
     
       21. The arrangement of  claim 20  wherein the pull-up and pull-down networks are switched dynamically between two impedance settings when commutating between drive and termination modes. 
     
     
       22. The arrangement of  claim 20  wherein each apparatus further comprises:
 first pre-drivers connected to the variable impedance pull-up network for configuring the pull-up network by sending a first control signal to the at least one first control input, the first pre-drivers being configured to operate with a second supply voltage higher than the first supply voltage; and 
 second pre-drivers connected to the variable impedance pull-down network for configuring the pull-down network by sending a second control signal to the at least one second control input, the second pre-drivers being configured to operate with the second supply voltage. 
 
     
     
       23. The arrangement of  claim 22 , wherein:
 for each I/O, each first pre-driver of the respective combined drive and termination circuit comprises AND-OR-AND logic comprising a first AND gate that receives a first input to indicate drive high, and a second AND gate that receives a third input to indicate termination; 
 for each I/O, each second pre-driver of the respective combined drive and termination circuit comprises AND-OR-AND logic comprising a third AND gate that receives a second input to indicate drive low, and a fourth AND gate that receives the third input to indicate termination; and 
 for each I/O, the first pre-drivers and the second pre-drivers having outputs that cause the variable impedance pull-up network and the variable impedance pull-down network to switch between two impedance settings. 
 
     
     
       24. The apparatus of  claim 16  further comprising a calibration logic that calibrates the impedances against an impedance reference. 
     
     
       25. An arrangement comprising:
 core logic; 
 a plurality of inputs each having a respective input pad, and a plurality of outputs each having a respective output pad; 
 for each input pad, a respective apparatus according to  claim 16  in which the combined drive and termination circuit is permanently configured to be in termination mode; and 
 for each output pad, a respective apparatus according to  claim 16  in which the combined drive and termination circuit is permanently configured to be in drive mode. 
 
     
     
       26. An apparatus according to  claim 16  further comprising:
 a controller that generates the control inputs as a function of whether the first combined drive and termination circuit is in a drive mode or a termination mode. 
 
     
     
       27. The apparatus of  claim 16  wherein:
 the plurality of transistors in the pull-up network are connected together in parallel; and 
 the plurality of transistors in the pull-down network are connected together in parallel. 
 
     
     
       28. An apparatus comprising:
 a first combined drive and termination circuit configured to operate with a first supply voltage, the first combined drive and termination circuit comprising:
 a variable impedance pull-up network; a variable impedance pull-down network; 
 at least one control input for setting a configuration of the pull-up network; and 
 at least one control input for setting a configuration of the pull-down network; and 
 
 a replica of at least part of the combined drive and termination circuit for use in performing calibration; 
 the pull-up network comprising a plurality of N-type mosfet transistors, and the pull-down network comprising a plurality of N-type mosfet transistors, the apparatus further comprising a controller that controls calibration being performed in two steps: 
 1) N device output impedance calibration to determine how many of the N-type transistors to enable for drive mode when a data output is logic low;
 2) N device termination calibration to determine how many of the N-type transistors to enable for termination; 
 
 the apparatus having a termination mode of operation in which the variable impedance pull-up network is configured to have a pull-up network termination impedance and the variable impedance pull-down network is configured to have a pull-down network termination impedance, the pull-up network and the pull-down network in combination functioning as a split termination; 
 the apparatus having a drive mode of operation in which:
 to drive a high output, the pull-up network is configured to generate a specific impedance when switched ON; 
 to drive a low output, the pull-down network is configured to generate a specific impedance when switched ON; 
 
 each of the pull-up network and the pull-down network having a plurality of transistors sized to provide identical impedances, and 
 each of the pull-up network and the pull-down network being configured to have a desired impedance by progressively varying the number of switched ON transistors. 
 
     
     
       29. An arrangement comprising:
 a plurality of apparatuses each comprising:
 a first combined drive and termination circuit configured to operate with a first supply voltage, the first combined drive and termination circuit comprising: 
 a variable impedance pull-up network; 
 a variable impedance pull-down network; 
 at least one control input for setting a configuration of the pull-up network; and 
 at least one control input for setting a configuration of the pull-down network; and 
 
 interconnections that pass common calibration values using one or more thermometer codes to each combined drive and termination circuit 
 each apparatus having a termination mode of operation in which the variable impedance pull-up network is configured to have a pull-up impedance and the variable impedance pull-down network is configured to have a pull-down network termination impedance, the pull-up network and the pull-down network in combination functioning as a split termination; 
 each apparatus having a drive mode of operation in which:
 to drive a high output, the pull-up network is configured to generate a specific impedance when switched ON; 
 to drive a low output, the pull-down network is configured to generate a specific impedance when switched ON; 
 
 each of the pull-up network and the pull-down network having a plurality of transistors sized to provide identical impedances, and 
 each of the pull-up network and the pull-down network being configured to have a desired impedance by progressively varying the number of switched ON transistors. 
 
     
     
       30. A method of providing combined drive and termination, the method comprising:
 in a termination mode of operation, configuring a variable impedance pull-up network forming part of a combined drive and termination circuit to have a pull-up network termination impedance and configuring a variable impedance pull-down network forming part of the combined drive and termination circuit to have a pull-down network termination impedance, the pull-up network and the pull-down network in combination functioning as a split termination; 
 in a drive mode of operation, to drive a high output, using first pre-drivers to configure the pull-up network to generate a first drive impedance; 
 in the drive mode of operation, to drive a low output, using second pre-drivers to configure the pull-down network to generate a second drive impedance; 
 wherein each of the pull-up network and the pull-down network has a plurality of transistors sized to provide identical impedances, and 
 configuring each of the pull-up network and the pull-down network to have a desired impedance by progressively varying the number of switched ON transistors; and 
 performing calibration to calibrate the pull-up termination impedance, the pull-down termination impedance, the first drive impedance and the second drive impedance; 
 wherein performing calibration comprises:
 calibrating the pull-up network for drive mode when a data output is logic high; 
 calibrating the pull-down network for drive mode when a data output is logic low; 
 calibrating the pull-up network for termination mode; and 
 calibrating the pull-down network calibration for termination mode. 
 
 
     
     
       31. The method of  claim 30  wherein the impedance of the pull-up network and the impedance of the pull-down network are configured using one or more thermometer codes. 
     
     
       32. The method of  claim 31  further comprising:
 converting a Gray code that encodes the number of switched ON transistors to a thermometer code of said one or more thermometer codes. 
 
     
     
       33. The method of  claim 31  wherein:
 configuring the pull-up network to have a pull-up termination impedance comprises selectively turning on a consecutive number of a plurality of transistors forming the pull-up network as determined by a thermometer code of said one or more thermometer codes; and 
 configuring the pull-down network to have a pull-down termination impedance comprises selectively turning on a consecutive number of a plurality of transistors forming the pull-down network as determined by a thermometer code of said one or more thermometer codes. 
 
     
     
       34. The method of  claim 30 :
 calibrating the pull-up network for drive mode when a data output is logic high produces a first calibration result; and 
 calibrating the pull-up network for termination mode produces a second calibration result; 
 the method further comprising:
 using the first calibration result to calibrate the pull-down network for drive mode when a data output is logic low; and 
 using the second calibration result to calibrate the pull-down network for termination mode.